Electric Utilities use electronic communication systems in combination with Protection Relays to monitor transmission line conditions and provide control change commands when the transmission lines experience fault conditions. These fault conditions require immediate action to avert wide scale power outages and damage to expensive substation equipment. These Relays require a means to communicate these protection control states and command changes. Various communication paths are used to transmit and receive these critical commands. These communication methods include (but are not limited to) Power Line Carrier (PLC), Audio Tone, Analog and Digital Microwave, Fiber Optics and Spread Spectrum Radio. The physical location of the communication equipment is primarily within a relay house of a utility substation, and therefore is exposed to excessive environmental conditions including excessively high voltages, especially during fault conditions.
Two of the communication paths, Power Line Carrier (30 kHz to 500 kHz) and Audio Tone Systems (300 Hz to 4000 Hz) often multiplex many channels (frequencies) on a single path. While many newer transmitters and receivers now have some type monitoring of their own signals and some also the path in general, it is believed that no monitoring apparatus presently exists to independently monitor the communication path, or has the ability to monitor any selectable frequency.
Power Line Carrier Communications used for Power System Protection utilize the Electric Utility transmission line as the communication path. Various components are used to convert the transmission line into a viable path for Power Line Carrier (PLC) frequencies. If these components are not aligned properly, the misalignment can cause conditions that will adversely affect the signal. One method of determining proper system alignment is measuring the reflected power or Standing Wave Ratio (SWR). Power Line Carrier Systems are adjusted for minimal reflected power to assure maximum power transfer across the transmission line. Many things connected to the Transmission line affect the reflected power of the PLC system and conditions are constantly changing. Monitoring of reflected power (SWR) is an effective way to assure that the changing conditions of the transmission line do not adversely affect the original alignment of the PLC System and render it ineffective when needed. It is believed that present monitoring apparatus cannot externally monitor various frequency selective reflected power (SWR) measurements and assign alarm or status limits as programmed by the user.
The majority of monitoring devices presently manufactured for the Electric Utility substations now incorporates the ability to time stamp events that occur within the associated monitoring device that can be synchronized with satellite clocks. The time stamped events provide recorded event timelines that can be used to evaluate, locate, and remedy the defect on the communication path of the utility substations system. However, no substation hardened communication monitoring devices are believed to exist with the capability to monitor and retrieve such time stamped events locally or to a remote control site.
The present inventors recognize that Spectral Analysis of the Power Line Carrier path provides an opportunity for the Electric Utilities to evaluate the integrity of their control (or Trip) frequencies during the actual fault. During fault conditions, excessive noise conditions exist that can achieve the same levels of the control frequencies. In-band spectral analysis provides the user the ability to determine the Signal to Noise Ratio (SNR) during those conditions and provide valuable information for remedying mis-operations or loss of carrier signals.